The present invention generally relates to cooling systems, More particularly, the invention is directed to cooling apparatuses and methods utilizing a mix of thermoelectric cooling with at least one other type of cooling.
Sub-ambient cooling is conventionally accomplished through gas/liquid vapor phase compression based refrigeration cycles using Freon type refrigerants to implement the heat transfers. Such refrigeration systems are used extensively for cooling human residences, foods, and vehicles. Sub-ambient cooling is also often used with major electronic systems such as mainframe server and workstation computers. Though vapor compression cooling can be very efficient, it does require significant moving hardware, including at a minimum, a compressor, a condenser, an evaporator, and related coolant transfer plumbing. As a result of the complexity and associated high cost, vapor compression cooling has not found material acceptance in small cooling applications, for example personal computers.
The fact that CMOS logic can operate materially faster as the temperature decreases has been well known for at least ten years. For example, if CMOS logic devices are operated at xe2x88x9250xc2x0 C., the performance is improved by 50 percent over room ambient temperature operation. Liquid nitrogen operating temperatures, in the range of xe2x88x92196xc2x0 C., have shown 200 percent performance improvements. Similar benefits have shown to accrue for integrated circuit wiring, where metal wiring resistances decrease by a factor of 2 for integrated circuits operated at xe2x88x9250xc2x0 C. in comparison to room ambient operation. This improvement rivals the recent technological breakthrough of using copper wiring in integrated circuits to reduce interconnect resistance and thereby effectively increase the operating frequencies attainable. Thus, sub-ambient operation of integrated circuit logic devices, such as field effect transistors, as well as the interconnect wiring can materially improve the integrated circuit performance, leaving the question of how to accomplish such cooling in the confines of an ever decreasing size and materially shrinking cost environment.
Thermoelectric cooling is one alternative that has found some usage given the compact size of the prevalently used Peltier devices. A Peltier device is fabricated from semiconductor material such as bismuth telluride or lead telluride. Though new materials are now being evaluated in various universities, they have yet to reach fruition. The commonly used Peltier materials exhibit very high electrical conductivity and relatively low thermal conductivity, in contrast to normal metals which have both high electrical and thermal conductivity. In operation the Peltier devices transport electrons from a cold sink, at temperature Tcold, to a hot sink, at temperature Thot, in response to an electric field formed across the Peltier device.
FIG. 1 schematically depicts a conventional Peltier type thermoelectric element (TE) 1 with DC power supply 2 created the electric field across TE 1 while at a load current 3. The desired heat transfer is from cold sink 4, at temperature Tcold, to hot sink 6, at temperature Thot. As indicated in the equation of FIG. 1, the net heat energy transported is composed of three elements, the first representing the Peltier effect (thermoelectric) contribution, the second defining negative Joule heating effects, and the third defining negative conductivity effects. The thermoelectric component is composed of the Seebeck coefficient, the temperature of operation (Tcold) and the current being applied. The Joule heating component reflects that roughly half the Joule heating goes to the cold sink and remainder to the hot sink. Lastly, the negative component attributable to thermal conduction represents the heat flow through the Peltier device, as defined by the thermal conductivity of the Peltier device, from the hot sink to the cold sink. See equation (1).
q=xcex1TcoldIxe2x88x92xc2xdI2Rxe2x88x92Kxcex94Txe2x80x83xe2x80x83(1)
Peltier device thermoelectric cooling is very reliable in that the cooling is entirely solid state. The key negative aspect of thermoelectric cooling is the inefficiency, wherein a Peltier device cooling system efficiency is commonly only in the range of 20 percent for a relatively nominal temperature drop between the cold sink and the ambient. Equation (1) above clearly shows how quickly the Peltier device becomes inefficient. Since the thermoelectric component of the heat transport increases in direct proportion to the current, while the Joule heating increases in proportion to the square of the current, the thermal conduction is in direct proportion to the hot sink to cold sink temperature difference. For example, to cool at the rate of one watt at a sub-ambient temperature of 0xc2x0 C., the Peltier cooling system must be powered with 5 watts. As the amount of heat to be transferred increases, the total power to be dissipated into the ambient mandates large convection devices and high output power supply circuits. Therefore, Peltier device thermoelectric cooling has poor efficiencies for large temperature differentials as compared to vapor compression cooling systems and as a result has not been considered a broadly applicable technology for improving integrated circuit performance.
Although vapor compression cooling systems are advantageous to current thermoelectric cooling configurations for sub-ambient cooling, such systems are not without limitations, especially for sub-zero centigrade cooling applications. Readily available vapor compression cooling systems are currently based on compressors and gas mixtures that are optimized for xe2x88x9220xc2x0 C. operation. An example of such a sub-zero centigrade vapor compression cooling system 200 is shown in FIG. 2. Vapor compression cooling system 200 includes compressor 201, condenser 202, and a coolant distribution system 203 including a counterflow heat exchanger 204, evaporator 205, JT valve 206, input line 207, and return line 208 all of which are well known in the art.
Vapor compression cooling system 200 is used to cool multi-chip modules (MCMs) 209 through providing a refrigerant in close proximity to MCMs 209. The entire coolant distribution system 203 and MCMs 209 are insulated by superinsulation 210. Superinsulation 210 and defrost control 211 limits the amount of condensation 212 produced as byproduct of utilizing vapor compression cooling system 200 for cooling to sub-zero centigrade temperatures.
As one attempts to achieve operating temperatures below xe2x88x9220xc2x0 C., there are numerous problems and limitations encountered with vapor compression cooling systems. For example, as the operating temperature is decreased, the volume for the compressor will increase causing space and weight limitations. Additionally, as the operating temperature is decreased the cost for cooling will increase. Cooling systems operating at temperatures below xe2x88x9220xc2x0 C. require increased insulation and defrost control to prevent further condensation 212 and condensation related reliability problems. A further limitation of vapor compression systems operating below xe2x88x9220xc2x0 C. is the inability to respond quickly to cooling demands. For example, advances in processing speeds of integrated circuits create fast temperature transients that require expedient cooling that current vapor compression systems can not accommodate.
The cost performance ratio of vapor compression cooling systems is determined by the cost and performance of compressors and fluids in the refrigeration cycle. Zero-centigrade vapor compression cooling systems can leverage the cost performance ratio by producing high volumes of zero-degree centigrade compressors utilizing standard refrigerant fluids (R134, etc.) for ubiquitous industry applications. However, to achieve temperatures below 0xc2x0 C., such as xe2x88x9250xc2x0 C., new compressors and fluids must be used. This added demand in performance significantly increases the cost performance ratio of current vapor compression cooling systems.
Thus, there are a number of very fundamental constraints on efficiency and differential temperature that limit the use of conventional cooling systems for sub-ambient cooling applications.
The present invention overcomes the fundamental individual constraints of conventional cooling systems and thermoelectric cooling by using thermoelectric coolers in conjunction with conventional cooling systems to optimize the operating temperature of thermoelectric coolers, thus creating an efficient cooling apparatus.
In one form, the invention relates to an apparatus comprising an object of a first nominal temperature, a cooling system of a second nominal temperature, the second nominal temperature being relatively greater than the first temperature, and a thermoelectric cooler situated to be coupled to the object and the cooling system.
In another form, the invention relates to a cooling apparatus operable in an ambient comprising a cooling system for absorbing thermal energy at a temperature T less than the ambient, and a thermoelectric cooler associated with the cooling system for providing a xcex94T between the cooling system and an object so as to provide a temperature of approximately Txe2x88x92xcex94T to the object.
In a still further form, the invention relates to a method of operating an apparatus having an object, a cooling system, and a thermoelectric cooler situated to be coupled to the object and the cooling system, comprising the steps of maintaining the object at a first nominal temperature, maintaining the cooling system at a second nominal temperature, wherein the second nominal temperature is relatively greater than the first temperature, and enabling the thermoelectric cooler.
In a still further form, the invention relates to a method of operating a cooling apparatus operable in an ambient having a cooling system, and a thermoelectric cooler associated with the cooling system comprising the steps of absorbing thermal energy at a temperature T less than the ambient from the thermoelectric cooler, providing, by the thermoelectric cooler, a temperature differential of xcex94T, and providing, to an object associated with the thermoelectric cooler, a temperature of approximately Txe2x88x92xcex94T.
In a particularized form of the invention, an apparatus provides a cooling system, such as a vapor phase cooling system coupled to a thermoelectric cooler including at least one Peltier device. The cooling system provides a temperature T thermally coupled to the thermoelectric cooler that provides a temperature change of xcex94T. The apparatus thus provides an overall temperature differential of approximately Txe2x88x92xcex94T to an object. Efficiency of the thermoelectric cooler is obtained through operating the thermoelectric cooler at sub-ambient temperatures as provided by the cooling system.
These and other features of the invention will be more clearly understood and appreciated upon considering the detailed embodiments described hereinafter.